Low-Noise Amplifiers (LNAs) are critical components of radio receivers. The primary purpose of an LNA is to provide sufficient gain to input signals such that the signal level is adequate for signal processing blocks downstream of the LNA. That is, the downstream signal processing blocks require a minimum signal level in order to effectively operate on inputs signals without drastically increasing the overall noise figure. In addition to providing sufficient gain, the LNA should process an input signal linearly and with a low noise figure. Further, the input impedance of an LNA should adequately match an upstream RF filter present in an RF front end of the radio receiver.
In addition to the above design criteria, an LNA must also meet the toughest criteria imposed by standards such as the Long-Term Evolution (LTE) Release 8 and Release 10 specifications.
A prior art solution to meet all of the above criteria is shown in FIG. 1, which is a schematic block diagram illustrating a conventional inductively degenerated common-source LNA. Typically, the input impedance of an LNA is designed to be a real value (e.g., 50 Ohms) that matches the output impedance of an RF front end. For this purpose an inductance Lg is introduced to cancel out the capacitive part (i.e., the imaginary part) of the input impedance at a desired operating frequency. The input impedance Zin of the LNA can be calculated by the following formula:
      z    in    =                              g          m                ·                  L          s                            C        gs              +          j      ⁡              [                              ω            ⁡                          (                                                L                  s                                +                                  L                  g                                            )                                -                      1                          ω              ⁢                                                          ⁢                              C                gs                                                    ]            
The real part of the input impedance is dependant upon the forward trans-conductance gm of transistor M1, capacitance Cgs and inductance Ls. Capacitor C is a DC blocking capacitor and resistor R is placed between Vbias and transistor Mt.
WO 07/006867 discloses a switchable symmetric shortcut at the certain location of a monolithic planar inductor whose inductance is practically distributed into smaller inductor portions. The smaller inductor portions are provided in a cascade configuration in a manner that causes inductor to function as a differential inductor device. In the configuration, an intermediate node between the (electrically) intermediate inductor portions forms common-mode point and the outer ends of the (electrically) outer inductor portions form differential-mode outputs of the differential inductor. Some of the inductor portions are arranged to be symmetrically bypassed or shortcut in relation to the common point in one or more steps for operation in one or more higher radio frequency bands. By means of the switchable symmetric shortcut, a controllable inductance step can be provided.
WO 07/085866 discloses an amplifier having multiple gain modes comprises a plurality of cascoded input transistors connected to an input and arranged in parallel, a degeneration stage connected to the input transistors and having a variable impedance, and switching means for switching between different modes of the amplifier by switching off one or more of the input transistors and varying the impedance of the degeneration stage.
Further related art can be found in WO 01/41302 and US Patent Publication No. 2008/0029753.
Reducing the bias current ID of transistor M1 affects the forward trans-conductance gm. When the transistor M1 operates in the saturation region, the following formula applies:gm∝√{square root over (ID)}
As the bias current is reduced, the transistor M1 leaves the saturation and active regions and enters the sub-threshold region, where:gm∝ID 
Thus the dependence of gm (and hence of the input impedance Zin) on ID strengthens as ID is reduced. This means that the real part of the input impedance Zin drops increasingly faster in relation to a linear reduction of ID.
This behaviour is a drawback because power consumption of an LNA accounts for a significant percentage of the total power budget for a radio receiver, and one conventional technique for reducing power consumption is to reduce the bias current of transistor M1.